The Federal Trade Commission is investigating whether roughly a dozen prominent advertising and advocacy groups violated antitrust law by coordinating boycotts among advertisers that did not want their brands to appear alongside hateful online content, three people familiar with the inquiries said.

The inquiry includes the agency’s previously reported investigation of Media Matters, a liberal advocacy organization that has published research on hateful and antisemitic content on X, the social media company owned by Elon Musk, along with other advocacy groups and advertising industry organizations.

Vanessa Otero, the chief executive of Ad Fontes Media, a media watchdog group, said the organization received a letter from the F.T.C. on May 20 demanding information about its business in relation to an investigation about “possible collusion.” Nearly a dozen other organizations also received such letters, two people familiar with the matter said.

The F.T.C.’s investigation into Media Matters, which is aligned with Democrats, required the organization to share copies of its budgets, documents showing the effects of “harmful” online content on advertisers and communications with other watchdog groups.

Advertisers have cut and frozen spending on X and several other conservative social media platforms in recent years as brands worried about appearing alongside inflammatory content.

Mr. Musk, who has been a close adviser to President Trump, sued Media Matters in 2023 over claims that it tried to damage X’s relationship with advertisers. That lawsuit continues.

The F.T.C. declined to comment, citing its practice of not commenting on active investigations.

This is a developing story. Check back for updates.